Monoethylene glycol (MEG) and monopropylene glycol (MPG) are valuable materials with a multitude of commercial applications, e.g. as heat transfer media, antifreeze, and precursors to polymers such as polyethylene terephthalate (PET).
Said glycols are currently made on an industrial scale by hydrolysis of the corresponding alkylene oxides, which are the oxidation products of ethylene and propylene, generally produced from fossil fuels.
In recent years increased efforts have been focussed on reducing the reliance on fossil fuels as a primary resource for the provision of fuels and commodity chemicals. Carbohydrates and related ‘biomass’ are seen as key renewable resources in the efforts to provide new fuels and alternative routes to desirable chemicals.
In particular, certain carbohydrates can be reacted with hydrogen in the presence of a catalyst system to generate polyols and sugar alcohols. Current methods for the conversion of saccharides to glycols revolve around a hydrogenation/hydrogenolysis process.
Reported processes generally require a first catalytic species to perform the hydrogenolysis reaction, which is postulated to have a retro-aldol mechanism, and a second catalytic species for hydrogenation.
Processes for the conversion of cellulose to products including MEG are described in Angew. Chem. Int. Ed. 2008, 47, 8510-8513 and Catalysis Today 147 (2009), 77-85 using nickel-promoted tungsten carbide catalysts.
US 2011/0312487 A1 describes a process for generating at least one polyol from a saccharide-containing feedstock and a catalyst system for use therein, wherein said catalyst system comprises a) an unsupported component comprising a compound selected from the group consisting of a tungsten compound, a molybdenum compound and any combination thereof; and b) a supported compound comprising an active metal component selected from the group consisting of Pt, Pd, Ru, Rh, Ni, Ir, and combinations thereof on a solid catalyst support.
Examples of the unsupported catalyst component in US 2011/0312487 A1 are said to include tungstic acid (H2WO4), ammonium tungstate ((NH4)10H2(W2O7)6), ammonium metatungstate ((NH4)6H2(W12O40).xH2O), ammonium paratungstate ((NH4)10[H2W12O42].4H2O), and tungstate, metatungstate and paratungstate compounds comprising at least Group I or II element.
Catalyst systems tested in US 2011/0312487 A1 utilise tungstic acid, tungsten oxide (WO2), phosphotungstic acid (H3PW12O40) and ammonium metatungstate as the unsupported catalyst component in conjunction with various nickel, platinum and palladium supported catalyst components.
US 2011/03046419 A1 describes a method for producing ethylene glycol from a polyhydroxy compound such as starch, hemicellulose, glucose, sucrose, fructose and fructan in the presence of catalyst comprising a first active ingredient and a second active ingredient, the first active ingredient comprising a transition metal selected from iron, cobalt, nickel, ruthenium, rhodium, palladium, iridium, and platinum, or a mixture thereof; the second active ingredient comprising a metallic state of molybdenum and/or tungsten, or a carbide, nitride, or phosphide thereof.
Angew. Chem. Int. Ed. 2012, 51, 3249-3253 describes a process for the selective conversion of cellulose into ethylene glycol and propylene glycol in the presence of a ruthenium catalyst and tungsten trioxide (WO3).
AIChE Journal, 2014, 60 (11), pp. 3804-3813 describes the retro-aldol condensation of glucose using ammonium metatungstate as catalyst.
Continuous processes for generating at least one polyol from a saccharide-containing feedstock are described in WO 2013/015955 A, CN 103731258 A and WO 2015/028398 A1.
The products of the afore-mentioned processes are typically a mixture of materials comprising MEG, MPG, 1,2-butanediol (1,2-BDO) and other by-products.
The reactor temperature selected in processes for the conversion of saccharide-containing feedstocks to glycols depends upon the nature of the saccharide-containing feedstock and is typically selected to achieve a good balance of retro-aldol activity which is favoured at higher temperatures and hydrogenation which is favoured at lowered temperatures.
Generally, said processes are typically performed at reactor temperatures within the range of from 195 to 245° C.
For example, when glucose is the starting saccharide, then typical reactor temperatures are in the range of from 195 to 230° C. When lower temperatures are employed, the sorbitol by-product yield from the hydrogenation of glucose increases and the yield of glycols decreases.
In order to effect energy savings, it is highly desirable to be able to utilise lower reactor temperatures without adversely affecting the yield of product glycols in the conversion of saccharide-containing feedstocks. Other benefits of lower reactor temperature include less of the starting material being converted to by-products and so there is a potential to further increase glycol yields. Another advantage would be to be able to operate at a lower hydrogen pressure as hydrogenation is favoured at lower temperature. Furthermore, lower temperature operation would also potentially result in lower metallurgy corrosion rates.